We have : 3aa1 is divisible by 9

<=> (3 + a + a + 1)  is divisible by 9

<=> (2a + 4)  is divisible by 9

<=>2(a + 2) is divisible by 9

<=> a + 2 is divisible by 9

<=> a = 7

        Answer 3771

Chào mừng các bạn tới đêm trăng sáng

When starting, the motorist goes in :

                      60 : 20 = 3 ( hours)

When to return, the motorist goes in :

                         60 : 30 = 2 ( hours )

In the entire trip, the motorist goes in :

                       3 + 2 = 5 ( hours )

Medium speed of the motorist for the entire trip :

                 60 : 5 = 12 (  miles per hour )

                                         Answer : 12 miles per hour

Mai has the stamp number is : 40 + 7 = 47 ( stamps )

                                Answer : 47 stamps

We have : AE = AD = 10 cm (because of the folding)

Applying the Pythagoras theorem to the right ΔABEΔABEΔABE,we have :

BE=√AE2−AB2=√100−64=√36=6BE=√AE2−AB2=√100−64=√36=6BE=AE2−AB2=100−64=36=6(cm)

=> EC = BC - BE = 10 - 6 = 4 (cm) 

Total height of 2 boys are: 131.6 x 2 =

263.2(cm)
Total height of 4 girls are: 128.3 x 4 =

513.2(cm)
Total height of all children are:

263.2 + 513.2 = 776.4(cm)
The average height of all children are:

776.4 : 6 = 129.4(cm)

                          Answer: 129.4cm

yes 100%

Applying the converse of Pythagoras theorem,we have :

(1) ⎧⎪ ⎪⎨⎪ ⎪⎩(√1)2+(√2)2=3(√3)2=3{(1)2+(2)2=3(3)2=3⇒(√1)2+(√2)2=(√3)2⇒(1)2+(2)2=(3)2

(2) {32+42=2552=25{32+42=2552=25=> 32 + 42 = 52

(3){112+122=263132=169{112+122=263132=169⇒112+122≠132⇒112+122≠132

(4){16652+22202=5552.32+5552.42=5552.2527752=5552.52=5552.25{16652+22202=5552.32+5552.42=5552.2527752=5552.52=5552.25

⇒16652+22202=27752⇒16652+22202=27752

Hence,the groups (1),(2),(4) can be the length of the sides of a right triangle

yes 100%

784 rectangle in group

Pigeonhole principle

Time upon the road as:

 9h42'-8h30'=1h12'

  1h12'=1,2h

Motor of motor are;

60:1,2=50(km/giờ)

    50km/giờ